Contemporary reciprocating engine systems include apparatus for detecting a knocking condition. Typically, a knocking condition occurs when an ignition event is too far advanced for an air fuel mixture charge in a cylinder in relation to the octane rating of the fuel.
Often, a knock detection system provides a signal to an ignition advance section of an engine control to indicate that a knock condition exists and directs that the ignition advance should be reduced or retarded. When there is a no-knock condition indicated, the ignition advance is slowly re-advanced until the knock limit is again exceeded and a knock re-occurs in one or several of the engine cylinders. It should be noted that in mounting a typical crankshaft position sensor to the engine, for use by the engine control's main ignition advance control section, there could be a slight error between the indicated engine crankshaft position and the actual engine crankshaft position. Thus, with the error in the crankshaft position sensor indication and the variation in octane rating of the fuel purchased and used in the engine, the engine control does, to know what the exact critical ignition advance knock limit is, in all cases and conditions. Knock detection systems can be used to compensate for this condition.
The purpose of a knock detection system is to identify when the engine enters into a knock condition so that the ignition advance section of the engine control can adaptively determine and update the ignition advance so as not to exceed the knock limit. Since the engine control is continuously re-determining the variations in the ignition advance knock limit from-information provided by the knock detection system, it is undesirable to have the detection system's operation dependent of a need to know how far the current ignition advance is from the knock limit. It should be noted that there are many possible levels of knock and that knock can occur prior to the critical ignition advance knock limit. In essence the probability of a knock occurrence and the probability of a magnitude of the knock is a function of spark advance. Also, in the calibration of a knock detection system, the calibration is deliberately set to allow some low level of knock to occur without indicating a knock condition to the ignition advance control.
The above description is important to consider since normal engine combustion noise, which shares a same frequency spectrum as a knocking condition, also increases as a function of increasing ignition advance. This occurs in the same ignition advance region as the critical ignition advance knock limit. It is therefore necessary in the design of a knock detection system to properly factor in the presence of combustion noise and not allow it to be adversely considered as an indication of a larger magnitude knock condition than is actually occurring in the engine.
Typical production acoustic or accelerometer knock sensors exhibit a wide production gain variation from sensor to sensor. In many cases the production sensor to sensor gain difference can exceed the difference between the magnitude of the mechanical noise read by a specific sensor and the magnitude of the knock occurrence read by the same sensor.
In order to properly and reliably use this type of production knock sensor, the detection system must first make a determination of the gain of the specific sensor or sensors being used on a specific engine. This is typically done by taking a reading of a known or predictable mechanical noise for that engine. In this case the variation in the sensor reading from the expected reading can be used as an indication of the sensor gain. It is desirable to establish this sensor gain indication so that the knock sensor only being be stimulated by the known or predictable mechanical engine noise and not be stimulated by a knock occurrence or combustion noise, both of which can vary in magnitude by the operation of the ignition advance control in response to the knock/no-knock indication provided by the knock detection system.
Once such a reliable mechanical engine noise times sensor gain reading is established, this value can be used in comparison to a sensor reading of mechanical engine noise plus combustion noise plus knock magnitude times sensor gain in such a manner that the sensor to sensor gain variation does not influence the knock/no-knock decision and indication from the knock detection system to the ignition advance control. However, should the sensor gain determination reading be an indication of the mechanical engine noise plus combustion noise plus a potential amount of low level knock times the sensor gain, then the variation from the expected sensor reading will not be a true indication of the pure sensor to sensor gain variation and the reliability of the use of this reading value as a sensor gain indication will be in question and can adversely effect a correct knock/no-knock decision and indication by the knock detection system.
Prior art knock detection schemes are inadequate because using a single integration window and a long term average approach combustion noise and low magnitude knock occurrence contribute to the sensor gain indication reading. Thus the long term average of the integration window reading when a no-knock decision is indicated, can vary as a function of ignition advance related combustion noise and knock occurrence and magnitude. The variation in this longer term average is not solely a function of sensor to sensor gain variation as is desired. Thus considering this long term average as an indication of sensor gain only, erroneous knock/no-knock decisions can result in the comparison method where it was assumed that the sensor gain variation was eliminated from the decision process.
What is needed is an improved knock detection system for a reciprocating engine that more reliably and accurately detects engine knocking over the full range of engine operating conditions. Such an improved knock detection system must improve the manner in which the sensor to sensor gain variation is determined.